1. Field of the Invention
The present invention relates generally to fibers formed of an inner core material of a first surface tension and an outer clad material of a second surface tension wherein the outer layer material adheringly surrounds the inner core material, and a method for producing the same.
The present invention is also directed to a compound formed by crosslinking the polyacrylonitrile and cyanate ester of the polyphenol of formula (I) and to uses of this crosslinked product in other composites.
2. Brief Discussion of Prior Art
Previous methods have been employed to form fibers from novolaks. Novolak fibers are generally used in fire retarding materials such as fireproof clothing, vehicle seating, blankets, electrical coverings, paints, vehicles, packaging, wastewater treatment and purification systems. Although a novolak fiber is thermally stable, the fibers have low structural stability.
Many methods for forming novolak resin fibers are known in the art. In one method, fibers are formed by melting a novolak and forming it into a fiber. A fiber is formed in a vessel having a heating element for heating the novolak within the vessel and an orifice at its bottom through which the novolak resin flows. To form the fiber, the novolak is placed within the vessel, the vessel is heated to a temperature sufficient to melt the novolak and maintain the novolak in the molten state, the molten novolak flows out of the orifice of the vessel and upon contacting the atmosphere and cooling, is wound upon a spool. Other known methods may be employed to form a novolak fiber. For example, the melted novolak may be extruded under pressure while being drawn and filtered under pressure to remove any solid impurities thereby improving the quality of the fibers. Optionally and instead of drawing, a blowing method can be employed such that the melted novolak is allowed to drop in a thin stream into a path of a blast of air which fiberizes the stream.
A general discussion of novaloid fiber forming techniques is found in Chemtech, July, 1989, p. 424. A discussion of pultrusion and melt spinning is available in Text Book of Polymer Science, 3rd, Chapters 17 and 18, John Wiley and Sons, 1984. Jetting technique is described in C. H. Hertz, Fluid Mech., Vol. 131, pp. 271-87 (1983).
The process of hardening or curing the fibers is very cumbersome. The curing mechanism generally involves the diffusion of formaldehyde into the fiber and reaction of the novolak and formaldehyde to bring about crosslinking of the novolak molecules. The curing process involves exposing the fibers to formaldehyde or other aldehydes in the presence of an acid or base catalyst. The fibers are exposed in one method by passing through an aqueous solution or by vapors these materials. Further, extensive washing of the fibers is often required to remove the acid. Furthermore, these traditional techniques have resulted in undesirable bi-products as in the case with formaldehyde or epoxy derivatives.
A major problem with the novolak fibers discussed above is that they are weak and brittle, although thermally stable. Blends or crosslinked fibers have been made from novolaks and a flammable but structurally stable fiber material to improve their strength. However, the problem of thermal instability arises because the flammable fiber component remains exposed to the atmosphere.
Side-by-side spinning techniques have been employed using phenolic resins in combination with a heat-meltable resin to form a fiber as described in U.S. Pat. No. 3,996,327. This technique has a drawback in that when the amount of the heat-meltable resin is increased, the mechanical strength increases, however, chemical and thermal stability decrease. Furthermore, side-by-side spinning does not result in crosslinking the heat-meltable fiber with the phenolic resin